High-K gate dielectric

ABSTRACT

Semiconductor devices and methods are provided. A semiconductor device according to the present disclosure includes a first transistor having a first gate dielectric layer, a second transistor having a second gate dielectric layer, and a third transistor having a third gate dielectric layer. The first gate dielectric layer includes a first concentration of a dipole layer material, the second gate dielectric layer includes a second concentration of the dipole layer material, and the third gate dielectric layer includes a third concentration of the dipole layer material. The dipole layer material includes lanthanum oxide, aluminum oxide, or yttrium oxide. The first concentration is greater than the second concentration and the second concentration is greater than the third concentration.

PRIORITY DATA

This application is a continuation application of U.S. patentapplication Ser. No. 16/901,340, filed Jun. 15, 2020, the entirety ofwhich is incorporated herein by reference.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapidgrowth. In the course of IC evolution, functional density (i.e., thenumber of interconnected devices per chip area) has generally increasedwhile geometry size (i.e., the smallest component (or line) that can becreated using a fabrication process) has decreased. This scaling downprocess generally provides benefits by increasing production efficiencyand lowering associated costs. However, such scaling down has also beenaccompanied by increased complexity in design and manufacturing ofdevices incorporating these ICs, and, for these advances to be realized,similar developments in device fabrication are needed.

As geometry size of IC devices continues to shrink, it becomes more andmore challenging to form features of the desirable shape. For example,in gate-last processes, a dummy gate stack is first formed as aplaceholder for a later-formed metal gate stack to undergo a substantialportion of the fabrication processes and then the dummy gate stack isremoved and replaced with the functional metal gate stack. To replacethe dummy gate stack, the dummy gate stack is first removed to form agate trench and then a plurality of layers is deposited in the gatetrench to form the functional metal gate stack. In some instances,additional dipole layers or dielectric layer may be deposited over thegate dielectric layer to provide transistors with different thresholdvoltages. Because the shrinking geometry also reduces the dimensions ofthe gate trenches, these additional dipole layers and dielectric layersmay reduce process window for satisfactorily depositing the plurality oflayers in the functional metal gate structure. In cases where the gatetrench is not straight, the process window may be further reduced.Therefore, while the conventional gate-last processes are adequate fortheir intended purposes, they are not satisfactory in all aspects.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1 illustrates a flowchart of a method of forming a semiconductordevice, according to aspects of the present disclosure.

FIGS. 2-15 are fragmentary cross-sectional views of a workpiece duringvarious operations of the method of FIG. 1 , according to aspects of thepresent disclosure.

FIG. 16 illustrates a flowchart of another method of forming asemiconductor device, according to aspects of the present disclosure.

FIGS. 17-27, 28A, and 28B are fragmentary cross-sectional views of aworkpiece during various operations of the method of FIG. 16 , accordingto aspects of the present disclosure.

FIG. 29 is a schematic circuit diagram of an 8-transistor (8T) staticrandom access memory (SRAM) cell, according to aspects of the presentdisclosure.

FIG. 30 is a schematic layout of the 8T SRAM cell in FIG. 29 , accordingto aspects of the present disclosure.

FIG. 31 is a schematic circuit diagram of a 10-transistor (10T) SRAMcell, according to aspects of the present disclosure.

FIG. 32 is a schematic layout of the 10T SRAM cell in FIG. 31 ,according to aspects of the present disclosure.

DETAILED DESCRIPTION

It is understood that the following disclosure provides many differentembodiments, or examples, for implementing different features of variousembodiments. Specific examples of components and arrangements aredescribed below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Forexample, the formation of a first feature over or on a second feature inthe description that follows may include embodiments in which the firstand second features are formed in direct contact, and may also includeembodiments in which additional features may be formed interposing thefirst and second features, such that the first and second features maynot be in direct contact. In addition, the present disclosure may repeatreference numerals and/or letters in the various examples. Thisrepetition is for the purpose of simplicity and clarity and does not initself dictate a relationship between the various embodiments and/orconfigurations discussed.

Moreover, the formation of a feature on, connected to, and/or coupled toanother feature in the present disclosure that follows may includeembodiments in which the features are formed in direct contact, and mayalso include embodiments in which additional features may be formedinterposing the features, such that the features may not be in directcontact. In addition, spatially relative terms, for example, “lower,”“upper,” “horizontal,” “vertical,” “above,” “over,” “below,” “beneath,”“up,” “down,” “top,” “bottom,” etc., as well as derivatives thereof(e.g., “horizontally,” “downwardly,” “upwardly,” etc.) are used for easeof the present disclosure of one features relationship to anotherfeature. The spatially relative terms are intended to cover differentorientations of the device including the features.

Still further, when a number or a range of numbers is described with“about,” “approximate,” and the like, the term is intended to encompassnumbers that are within a reasonable range including the numberdescribed, such as within +/−10% of the number described or other valuesas understood by person skilled in the art. For example, the term “about5 nm” encompasses the dimension range from 4.5 nm to 5.5 nm.

Metal gate stacks, which are commonplace in modern-day transistors, maybe formed using a gate-first process or a gate-last process. In theformer, the functional metal gate stack is formed before formation ofseveral features, such as source/drain features and an interlayerdielectric layer. In the latter, a non-functional dummy gate stack isfirst formed as a placeholder for a later-formed metal gate stack toundergo fabrication processes for the source/drain feature and theinterlayer and then the dummy gate stack is removed and replaced withthe functional metal gate stack. To replace the dummy gate stack, thedummy gate stack is first removed to form a gate trench and then aplurality of layers is deposited in the gate trench to form thefunctional metal gate stack. In some instances, one or more dipolelayers may be deposited over the gate dielectric layer to providetransistors with different threshold voltages. An interface dipole maybe created at the interface between a dipole layer and an underlyingsilicon oxide interfacial layer due to oxygen atom density differential.Depending on the polarity of the interface dipole, the interface dipolemay increase or reduce the threshold voltage of the subject transistor.While a dipole layer may be used to modulate the threshold voltage, anadditional dipole layer may further reduce the process windows forsatisfactorily depositing the plurality of layers in the functionalmetal gate stack. In cases where the gate trench is not straight, theprocess window may be further reduced. For example, before deposition ofall the plurality of layers in the gate trench to form the functionalmetal gate stack, two sides of a layer may merge and close the openingof the trench, preventing deposition of subsequent layers in the gatetrench.

To alleviate the foregoing problems, the present disclosure provides amethod of forming a semiconductor device. In some embodiments of thepresent disclosure, the method includes depositing a gate dielectriclayer over gate trenches in a first device region, a second deviceregion, and a third device region on a workpiece. A first dipole layeris then deposited over the gate dielectric layer in gate trenches in afirst device region, a second device region, and a third device region.The first dipole layer over the gate trench in the second device regionis selectively removed while the gate dielectric layer in the firstdevice region and the third device region remain covered by the firstdipole layer. A second dipole layer is then deposited over the firstdipole layer in the gate trenches in the first device region and thethird device region, as well as the gate dielectric layer in the seconddevice region. The first dipole layer and the second dipole layer arethen removed from over the gate dielectric layer in the third deviceregion. The workpiece is then annealed at a temperature between about500° C. and about 900° C. such that ingredients in the first dipolelayer and the second dipole layer may thermally diffuse into the gatedielectric layer to alter threshold voltages. After the first dipolelayer and the second dipole layer are then removed from the gatetrenches of the workpiece, a substantially identical gate structure isformed in the gate trenches in the first device region, the seconddevice region, and the third device region to form a first transistor inthe first device region, a second transistor in the second deviceregion, and a third transistor in the third device region. The firsttransistor in the first device region, the second transistor in thesecond device region, and the third transistor in the third deviceregion have different threshold voltages. Because the dipole layers arenot included in the gate trenches and only serve as vehicles fordiffusive dopants, the process window for forming the functional metalgate stack is not reduced. Embodiments having four device regions arealso provided.

FIG. 1 illustrates a flow chart of a method 100 for forming asemiconductor device according to various aspects of the presentdisclosure. FIGS. 2-15 are fragmentary cross-sectional views of aworkpiece at various stages of fabrication of the method 100 in FIG. 1 .Additional steps can be provided before, during, and after method 100,and some of the steps described can be moved, replaced, or eliminatedfor additional embodiments of method 100. Additional features can beadded in the contact structure depicted in FIGS. 2-15 , and some of thefeatures described below can be replaced, modified, or eliminated inother embodiments of the interconnect structure depicted in FIGS. 2-15 .

Referring to FIGS. 1 and 2 , the method 100 includes a block 102 where aworkpiece 200 is received. Upon conclusion of the method 100, theworkpiece 200 may be fabricated into a semiconductor device 200. In thatsense, the workpiece 200 may also be referred to as a semiconductordevice 200 in suitable context. The semiconductor device 200 can beincluded in a microprocessor, a memory, and/or other IC device. In someimplementations, the semiconductor device 200 is a portion of an ICchip, a system on chip (SoC), or portion thereof, that includes variouspassive and active microelectronic devices, such as resistors,capacitors, inductors, diodes, p-type field effect transistors (PFETs),n-type field effect transistors (NFETs), metal-oxide semiconductor fieldeffect transistors (MOSFETs), complementary metal-oxide semiconductor(CMOS) transistors, bipolar junction transistors (BJTs), laterallydiffused MOS (LDMOS) transistors, high voltage transistors, highfrequency transistors, other suitable components, or combinationsthereof. The transistors may be planar transistors or multi-gatetransistors, such as fin-like FETs (FinFETs) or gate-all-around (GAA)transistors.

As illustrated in FIG. 2 , the semiconductor device 200 includes asubstrate (wafer) 202. In the depicted embodiment, substrate 202includes silicon. Alternatively or additionally, substrate 202 includesanother elemental semiconductor, such as germanium; a compoundsemiconductor, such as silicon carbide, gallium arsenide, galliumphosphide, indium phosphide, indium arsenide, and/or indium antimonide;an alloy semiconductor, such as silicon germanium (SiGe), GaAsP, AlInAs,AlGaAs, GaInAs, GaInP, and/or GaInAsP; or combinations thereof. In someimplementations, substrate 202 includes one or more group III-Vmaterials, one or more group II-IV materials, or combinations thereof.In some implementations, substrate 202 is a semiconductor-on-insulatorsubstrate, such as a silicon-on-insulator (SOI) substrate, a silicongermanium-on-insulator (SGOI) substrate, or a germanium-on-insulator(GOI) substrate. Semiconductor-on-insulator substrates can be fabricatedusing separation by implantation of oxygen (SIMOX), wafer bonding,and/or other suitable methods. Substrate 202 can include various dopedregions (not shown) configured according to design requirements ofsemiconductor device 200, such as p-type doped regions, n-type dopedregions, or combinations thereof. P-type doped regions (for example,p-type wells) include p-type dopants, such as boron, indium, otherp-type dopant, or combinations thereof. N-type doped regions (forexample, n-type wells) include n-type dopants, such as phosphorus,arsenic, other n-type dopant, or combinations thereof. In someimplementations, substrate 202 includes doped regions formed with acombination of p-type dopants and n-type dopants. The various dopedregions can be formed directly on and/or in substrate 202, for example,providing a p-well structure, an n-well structure, a dual-wellstructure, a raised structure, or combinations thereof. An ionimplantation process, a diffusion process, and/or other suitable dopingprocess can be performed to form the various doped regions.

Semiconductor device 200 includes an active region 204 over substrate202. The active region 204 may be a fin-shape semiconductor feature, ora vertical stack of nanostructures. As shown in FIG. 2 , the activeregion 204 horizontally extends along the X-direction over the substrate202 and extends vertically along the Z-direction from the substrate 202.In some implementations, the active region 204 may be a portion ofsubstrate 202 (such as a portion of a material layer of substrate 202).For example, silicon active region 204 may be formed from siliconsubstrate 202. Alternatively, in some implementations, the active region204 is defined in a material layer, such as one or more semiconductormaterial layers formed over substrate 202. For example, the activeregion 204 can include a semiconductor layer stack having varioussemiconductor layers (such as a heterostructure) disposed over substrate202. The semiconductor layers can include any suitable semiconductormaterials, such as silicon, germanium, silicon germanium, other suitablesemiconductor materials, or combinations thereof. The semiconductorlayers can include same or different materials, etching rates,constituent atomic percentages, constituent weight percentages,thicknesses, and/or configurations depending on design requirements ofthe semiconductor device 200. In some implementations, the semiconductorlayer stack includes alternating semiconductor layers, such assemiconductor layers composed of a first material and semiconductorlayers composed of a second material. For example, the semiconductorlayer stack alternates silicon layers and silicon germanium layers (forexample, SiGe/Si/SiGe/Si/SiGe/Si from bottom to top). In someimplementations, the semiconductor layer stack includes semiconductorlayers of the same material but with alternating constituent atomicpercentages, such as semiconductor layers having a constituent of afirst atomic percent and semiconductor layers having the constituent ofa second atomic percent. For example, the semiconductor layer stackincludes silicon germanium layers having alternating silicon and/orgermanium atomic percentages (for example,Si_(a)Ge_(b)/Si_(c)Ge_(d)/Si_(a)Ge_(b)/Si_(c)Ge_(d)/Si_(a)Ge_(b)/Si_(c)Ge_(d)from bottom to top, where a, c are different atomic percentages ofsilicon and b, d are different atomic percentages of germanium).

The active region 204 may be formed over substrate 202 by any suitableprocess. In some implementations, a combination of deposition,lithography and/or etching processes are performed to define the activeregion 204 illustrated in FIG. 2 . For example, forming the activeregion 204 includes performing a lithography process to form a patternedresist layer over substrate 202 (or a material layer, such as aheterostructure, disposed over substrate 202) and performing an etchingprocess to transfer a pattern defined in the patterned resist layer tosubstrate 202 (or the material layer, such as the heterostructure,disposed over substrate 202). The lithography process can includeforming a resist layer on substrate 202 (for example, by spin coating),performing a pre-exposure baking process, performing an exposure processusing a mask, performing a post-exposure baking process, and performinga developing process. During the exposure process, the resist layer isexposed to radiation energy (such as ultraviolet (UV) light, deep UV(DUV) light, or extreme UV (EUV) light), where the mask blocks,transmits, and/or reflects radiation to the resist layer depending on amask pattern of the mask and/or mask type (for example, binary mask,phase shift mask, or EUV mask), such that an image is projected onto theresist layer that corresponds with the mask pattern. Since the resistlayer is sensitive to radiation energy, exposed portions of the resistlayer chemically change, and exposed (or non-exposed) portions of theresist layer are dissolved during the developing process depending oncharacteristics of the resist layer and characteristics of a developingsolution used in the developing process. After development, thepatterned resist layer includes a resist pattern that corresponds withthe mask. The etching process uses the patterned resist layer as an etchmask to remove portions of substrate 202 (or a material layer disposedover substrate 202). The etching process can include a dry etchingprocess (for example, a reactive ion etching (RIE) process), a wetetching process, other suitable etching process, or combinationsthereof. After the etching process, the patterned resist layer isremoved from substrate 202, for example, by a resist stripping process.Alternatively, the active region 204 is formed by a multiple patterningprocess, such as a double patterning lithography (DPL) process (forexample, a lithography-etch-lithography-etch (LELE) process, aself-aligned double patterning (SADP) process, a spacer-is-dielectric(SID) SADP process, other double patterning process, or combinationsthereof), a triple patterning process (for example, alithography-etch-lithography-etch-lithography-etch (LELELE) process, aself-aligned triple patterning (SATP) process, other triple patterningprocess, or combinations thereof), other multiple patterning process(for example, self-aligned quadruple patterning (SAQP) process), orcombinations thereof. In some implementations, directed self-assembly(DSA) techniques are implemented while forming the active region 204.Further, in some implementations, the exposure process can implementmaskless lithography, electron-beam (e-beam) writing, ion-beam writing,and/or nanoimprint technology for patterning the resist layer and/orother layers.

An isolation feature (not shown) is formed over and/or in substrate 202to isolate various regions, such as various device regions, of thesemiconductor device 200. For example, the isolation feature separatesand isolates the active region 204 from an adjacent active region. Insome embodiment, the isolation feature may surround a bottom portion ofthe active region 204 and expose a top portion of the active region 204.The isolation feature may include silicon oxide, silicon nitride,silicon oxynitride, other suitable isolation material (for example,including silicon, oxygen, nitrogen, carbon, or other suitable isolationconstituent), or combinations thereof. The isolation feature can includedifferent structures, such as shallow trench isolation (STI) structures,deep trench isolation (DTI) structures, and/or local oxidation ofsilicon (LOCOS) structures. In some implementations, STI features can beformed by etching a trench in substrate 202 (for example, by using a dryetch process and/or wet etch process) and filling the trench withinsulator material (for example, by using a chemical vapor depositionprocess or a spin-on glass process). A chemical mechanical polishing(CMP) process may be performed to remove excessive insulator materialand/or planarize a top surface of isolation feature. In someimplementations, STI features can be formed by depositing an insulatormaterial over substrate 202 after forming the active region 204 (in someimplementations, such that the insulator material layer fills gaps(trenches) between the active region 204 and an adjacent active region)and etching back the insulator material layer to form isolation feature.In some implementations, the isolation feature includes a multi-layerstructure that fills trenches, such as a bulk dielectric layer disposedover a liner dielectric layer, where the bulk dielectric layer and theliner dielectric layer include materials depending on designrequirements (for example, a bulk dielectric layer that includes siliconnitride disposed over a liner dielectric layer that includes thermaloxide).

In embodiments illustrated in FIG. 2 , the workpiece 200 includes adummy gate stack 208 over and around a channel region 10 in the activeregion 204. Each of the dummy gate stack 208 may include a dummy gatedielectric layer 205 and a dummy gate electrode 207. In the embodimentsrepresented in FIG. 2 , the dummy gate stack 208 includes a dummy gatedielectric layer 205 over the active region 204 and a dummy gateelectrode 207 over the dummy gate dielectric layer 205. In someimplementations, the dummy gate dielectric layer 205 may be formed ofsilicon oxide and the dummy gate electrode 207 may be formed ofpolysilicon. A gate spacer layer 210 may be formed along sidewalls ofthe dummy gate stack 208 by any suitable process and include adielectric material. The dielectric material for the gate spacer layer210 may include silicon, oxygen, carbon, nitrogen, other suitablematerial, or combinations thereof (for example, silicon oxide, siliconnitride, silicon oxynitride, or silicon carbide). In someimplementations, the gate spacer layer 210 may include a multi-layerstructure, such as a first spacer layer that includes silicon nitrideand a second spacer layer that includes silicon oxide. In someimplementations, the gate spacer layer 210 may include more than one setof spacers, such as seal spacers, offset spacers, sacrificial spacers,dummy spacers, and/or main spacers, formed adjacent to the gate stacks.It is noted that the cross-sectional view in FIG. 2 depicts a crosssection that cut across a top surface of the active region 204 along theX-direction. For example, when the active region 204 is a fin-shapeactive region (or a fin), the dummy gate stack 208 in FIG. 2 are shownto be disposed on a top surface of the fin.

The workpiece 200 may further include source/drain features 212 formedin source/drain regions 20 adjacent the channel region 10. As shown inFIG. 2 , the source/drain features 212 are disposed adjacent the dummygate stack 208. In some implementations, source/drain features 212 areformed over source/drain regions 20 of the active region 204 using anepitaxy process. The epitaxy process can implement CVD depositiontechniques (for example, vapor-phase epitaxy (VPE), ultra-high vacuumCVD (UHV-CVD), LPCVD, and/or PECVD), molecular beam epitaxy, othersuitable SEG processes, or combinations thereof. Source/drain features212 may be doped with n-type dopants and/or p-type dopants. In someimplementations, where the transistor is configured as an n-type device(for example, having an n-channel), source/drain features 212 can besilicon-containing epitaxial layers or silicon-carbon-containingepitaxial layers doped with phosphorus, other n-type dopant, orcombinations thereof (for example, forming Si:P epitaxial layers orSi:C:P epitaxial layers). In some implementations, where the transistoris configured as a p-type device (for example, having a p-channel),source/drain features 212 can be silicon-and-germanium-containingepitaxial layers doped with boron, other p-type dopant, or combinationsthereof (for example, forming Si:Ge:B epitaxial layers). In someimplementations, annealing processes are performed to activate dopantsin source/drain features 212 of the semiconductor device 200.

The workpiece 200 may include a contact etch stop layer (CESL) 214deposited on the source/drain features 212. In some instances, the CESL214 may also be deposited over the side surfaces of the gate spacerlayer 210. In some embodiments, the CESL 214 may be formed of siliconnitride, silicon oxynitride, or silicon carbonitride and may beconformally deposited using atomic layer deposition (ALD). A dielectriclayer 216, which may also be referred to as an interlayer dielectric(ILD) layer 216, is deposited over the CESL 214. The dielectric layer216 may be formed using a flowable chemical vapor deposition (FCVD)process. In some implementations, after deposition using a FCVD process,the deposited dielectric layer 216 may be cured by incidence ofultraviolet (UV) radiation, annealing, or both. In some embodiments, thedielectric layer 216 may include a dielectric material including, forexample, silicon oxide, TEOS formed oxide, PSG, BPSG, low-k dielectricmaterial, other suitable dielectric material, or combinations thereof.Exemplary low-k dielectric materials include FSG, carbon doped siliconoxide, Black Diamond® (Applied Materials of Santa Clara, California),Xerogel, Aerogel, amorphous fluorinated carbon, Parylene, BCB, SiLK (DowChemical, Midland, Michigan), polyimide, other low-k dielectricmaterial, or combinations thereof.

Referring to FIGS. 1 and 3 , the method 100 includes a block 104 wherethe dummy gate stack 208 is removed to form a gate trench 218. In someembodiments, the dummy gate stack 208 may be removed using a combinationof suitable dry etch processes and wet etch processes to form the gatetrench 218. The gate trench 218 exposes the channel region 10 of theactive region 204. While the dummy gate stack 208 and the gate trench218 are depicted as having straight sidewalls along the Z direction,they may not be straight and may include a necking profile in someimplementations. The necking profile may reduce the process window todeposit a plurality of layers in the gate trench 218 to form thefunctional metal gate stack.

Referring to FIGS. 1 and 4 , the method 100 includes a block 106 wherean interfacial layer 220 is deposited in the gate trench 218. In someimplementations, the interfacial layer 220 may be formed of siliconoxide.

Referring to FIGS. 1 and 5 , the method 100 includes a block 108 where agate dielectric layer 222 is deposited over the interfacial layer 220.The gate dielectric layer 222 may include dielectric materials having ahigh dielectric constant, for example, greater than a dielectricconstant of silicon oxide (k≈3.9). Exemplary high-k dielectric materialsinclude hafnium, zirconium, tantalum, titanium, oxygen, nitrogen, othersuitable constituent, or combinations thereof. In some implementations,the gate dielectric layer 222 may include includes a high-k dielectricmaterial including, for example, HfO₂, HfSiO, HfSiON, HfTaO, HfTiO,HfZrO, ZrO₂, TiO₂, Ta₂O₅, other suitable high-k dielectric material, orcombinations thereof.

As will be described below, the workpiece 200 may include multipledevice regions (such as three device regions, four device regions, ormore device regions) for transistors having different threshold voltagesand the structure shown in FIGS. 2-5 may be repeated across thesemultiple device regions but the repeated structures are omitted, whichis representatively shown by ellipses (“ . . . ”) in FIGS. 2-5 .Operations of the method 100 that treat different device regionsdifferently will be described below in conjunction with FIGS. 6-15 . Forsimplicity and clarity of descriptions, FIGS. 6-14 illustratefragmentary cross-sectional views of area “I” in different deviceregions.

Referring to FIGS. 1 and 6 , the method 100 includes a block 110 where afirst dipole layer 224 is deposited over the gate dielectric layer 222.In the embodiments illustrated in FIGS. 6-15 , the workpiece 200includes three device regions—a first device region 1100, a seconddevice region 1200, and a third device region 1300. As described above,FIG. 6 illustrates the fragmentary cross-sectional views of the area “I”in the first device region 1100, the second device region 1200, and thethird device region 1300. The first dipole layer 224 is deposited on thegate dielectric layer 222 in the gate trenches in the first deviceregion 1100, the second device region 1200, and the third device region1300. In some embodiments, the first dipole layer 224 may be formed oflanthanum oxide, yttrium oxide, or aluminum oxide and may be depositedusing atomic layer deposition (ALD). In some implementations, the ALDprocess used to form the first dipole layer 224 may include betweenabout 2 and about 10 cycles. In those implementations, the first dipolelayer 224 may have a thickness between about 1 Å and about 10 Å. In oneembodiment, the first dipole layer 224 may be formed of lanthanum oxide.

Referring to FIGS. 1, 7, 8, and 9 , the method 100 includes a block 112where the first dipole layer 224 is selectively removed from the seconddevice region 1200. In some embodiments, photolithography techniques andetch techniques may be used to perform the operations at block 112. Anexample process is shown in FIGS. 7-9 . Reference is first made to FIG.7 . A hard mask layer 226 is first formed over the first dipole layer224 and a bottom antireflective coating (BARC) layer 228 is depositedover the hard mask layer 226. In some instances, the hard mask layer 226may be a single layer or a multi-layer. When the hard mask layer 226 isa single layer, the hard mask layer 226 may include silicon oxide,silicon nitride, or silicon oxynitride. When the hard mask layer 226 isa multi-layer, the hard mask layer 226 may include a silicon layer and asilicon nitride layer on the silicon layer. The BARC layer 228 mayinclude silicon oxynitride, a polymer, or a suitable material. Topattern the BARC layer 228 and the hard mask layer 226, a photoresistlayer may be blanketly deposited over the workpiece 200, including overthe BARC layer 228 in the first device region 1100, the second deviceregion 1200, and the third device region 1300. The photoresist layer maybe a single layer or a multi-layer, such as a tri-layer. The photoresistlayer is then exposed to radiation going through or reflected from amask, baked in a post-bake process, and developed in a developersolution to form a patterned photoresist mask. The BARC layer 228 andthe hard mask layer 226 are then patterned using the patternedphotoresist mask to form an etch mask having an opening over the seconddevice region 1200. The etch mask is then used in an etch process toselectively etch away the first dipole layer 224 in the gate trench inthe second device region 1200, as illustrated in FIG. 8 . The etchprocess may be a dry etch process, a wet etch process, or a suitableetch process. Referring to FIG. 9 , after the first dipole layer 224 isselectively removed from the gate trench in the second device region1200, the hard mask layer 226 and the BARC layer 228 in the first deviceregion 1100 and the third device region 1300 are removed using asuitable etching process.

Referring to FIGS. 1 and 10 , the method 100 includes a block 114 wherea second dipole layer 230 is deposited over the workpiece 200. As shownin FIG. 10 , the second dipole layer 230 is deposited on the firstdipole layer 224 in the gate trenches in the first device region 1100and the third device region 1300 as well as on the gate dielectric layer222 in the gate trench in the second device region 1200. In someembodiments, the second dipole layer 230 and the first dipole layer 224may have the same composition. Similar to the first dipole layer 224,the second dipole layer 230 may also be formed of lanthanum oxide,yttrium oxide, or aluminum oxide and may be deposited using atomic layerdeposition (ALD). In some implementations, the ALD process used to formthe second dipole layer 230 may include between about 2 and about 10cycles. In those implementations, the second dipole layer 230 may have athickness between about 1 Å and about 10 Å. In one embodiment, thesecond dipole layer 230 may be formed of lanthanum oxide.

Referring to FIGS. 1, 11, 12, and 13 , the method 100 includes a block116 where the second dipole layer 230 is selectively removed from thethird device region 1300. Similar to operations at block 112, operationsat block 116 may also be performed using photolithography and etchtechniques. For example, a hard mask layer 232 and a BARC layer 234 maybe formed over the second dipole layer 230, as shown in FIG. 11 . As thehard mask layer 232 may be similar to the hard mask layer 226 and theBARC layer 234 may be similar to the BARC layer 228, detaileddescriptions of the hard mask layer 232 and the BARC layer 234 areomitted for brevity. Thereafter a photoresist layer may then bedeposited over the BARC layer 234. The photoresist layer, the BARC layer234, and the hard mask layer 232 are then patterned in fashions similarto those described with respect to block 112 and will not be repeatedhere. The patterned hard mask layer 232 allows selective removal of thefirst dipole layer 224 and the second dipole layer 230 in the gatetrench in the third device region 1300, exposing the gate dielectriclayer 222 in the gate trench in the third device region 1300. Asillustrated in FIG. 12 , at this point, the gate trench in the firstdevice region 1100 includes thereover the first dipole layer 224 and thesecond dipole layer 230; the gate trench in the second device region1200 includes thereover the second dipole layer 230; and the gate trenchin the third device region 1300 is free of the first dipole layer 224and the second dipole layer 230. Put differently, the total thickness(along the Z direction) of the first dipole layer 224 and the seconddipole layer 230 in the gate trench in the first device region 1100 isgreater than the total thickness of the second dipole layer 230 in thegate trench in the second device region 1200, while the gate trench inthe third device region 1300 is free of any dipole layer. After thefirst dipole layer 224 and the second dipole layer 230 are selectivelyremoved from the gate trench in the third device region 1300, the hardmask layer 232 and the BARC layer 234 may then be removed from the firstdevice region 1100 and the second device region 1200, as illustrated inFIG. 13 .

Referring to FIGS. 1 and 13 , the method 100 includes a block 118 wherethe workpiece 200 is annealed in an anneal process 300. At block 118,the anneal process 300 is used to thermally drive elements in the firstdipole layer 224 and/or the second dipole layer 230 into the gatedielectric layer 222 in the gate trenches in the first device region1100 and the second device region 1200. The first dipole layer 224 andthe second dipole layer 230 serve as a diffusion doping vehicle to bringits elements to be in direct contact with the gate dielectric layer 222.The anneal process 300 may be a rapid thermal anneal (RTA) process, alaser spike anneal process, a flash anneal process, or a furnace annealprocess. In some implementation, the anneal process 300 includes a highanneal temperature between about 500° C. and about 900° C. so as toallow lanthanum, yttrium, or aluminum in the first dipole layer 224and/or the second dipole layer 230 to diffuse into the gate dielectriclayer 222 in gate trenches in the first device region 1100 and thesecond device region 1200. Because the gate trench in the third deviceregion 1300 is free of any dipole layer, the anneal process 300 at block118 does not result in any dipole layer material diffusing into the gatedielectric layer 222 in the third device region 1300. In someimplementations, the anneal process 300 may last between about 5 secondsand about 20 seconds.

Referring to FIGS. 1 and 14 , the method 100 include a block 120 wherethe first dipole layer 224 and the second dipole layer 230 are removedfrom the workpiece 200. After the element in the first dipole layer 224and the second dipole layer 230 is thermally driven into the gatedielectric layer 222 at block 118, the first dipole layer 224 and thesecond dipole layer 230 are removed from the gate trench in the firstdevice region 1100 and the second dipole layer 230 is removed from thegate trench in the third device region 1300. The operations at block 120may be performed using a dry etch process, a wet etch process, or asuitable etch process. As shown in FIG. 14 , the anneal process 300 atblock 118 results in a first gate dielectric layer 2221 in the gatetrench in the first device region 1100 and a second gate dielectriclayer 2222 in the gate trench in the second device region 1200. Due tolack of any dipole layer, the gate dielectric layer 222 in the gatetrench in the third device region 1300 remains substantially unchanged.

It has also been observed that a thicker dipole layer contributes to agreater doping concentration of the dipole layer material in the gatedielectric layer 222. For example, when the gate dielectric layer 222 isformed of hafnium oxide and the first dipole layer 224/second dipolelayer 230 is formed of lanthanum oxide, operations at block 118 mayresult in a first lanthanum concentration in the first gate dielectriclayer 2221 in the first device region 1100 and a second lanthanumconcentration in the second gate dielectric layer 2222 in the seconddevice region 1200. Because the gate trench in the third device region1300 is free of any dipole layer, the gate dielectric layer 222 in thegate trench in the third device region 1300 includes a third lanthanumconcentration that is zero. Because the gate trench in the first deviceregion 1100 includes both the first dipole layer 224 and the seconddipole layer 230, the first lanthanum concentration is greater than thesecond lanthanum concentration, which is greater than the zero thirdlanthanum concentration. Each of the first, second and third lanthanumconcentration may be represented as a ratio of lanthanum concentration(i.e., [lathanum] or [La] from the first dipole layer 224/second dipolelayer 230) to hafnium (i.e., [Hafnium] or [Hf] in the gate dielectriclayer 222). In the example described above, the first lanthanumconcentration (i.e., first lanthanum to hafnium ratio) may be about 0.4([La]/[Hf]) and the second lanthanum concentration (i.e., secondlanthanum to hafnium ratio) may be about 0.2 ([La]/[Hf]), while thethird lanthanum concentration (i.e., third lanthanum to hafnium ratio)is zero. The foregoing description generally applies to other dipolelayer materials, such as yttrium and aluminum, and other gate dielectricmaterial, such as HfSiO, HfSiON, HfTaO, HfTiO, HfZrO, ZrO₂, TiO₂, Ta₂O₅,provided that different dipole layer material may have differentdiffusivity and different dipole layer may have different solidsolubility in different gate dielectric layers.

Referring to FIGS. 1 and 15 , the method 100 includes a block 122 wherefurther processes are performed to form a first transistor 410 in thefirst device region 1100, a second transistor 420 in the second deviceregion 1200, and a third transistor 430 in the third device region 1300.As shown in FIG. 15 , the first transistor 410 includes the first gatedielectric layer 2221, the second transistor 420 includes the secondgate dielectric layer 2222, and the third transistor 430 includes thegate dielectric layer 222. Each of the first transistor 410, the secondtransistor 420, and the third transistor 430 includes a functional metalgate stack 236 in addition to the interfacial layer 220 and the firstgate dielectric layer 2221/second gate dielectric layer 2222/gatedielectric layer 222. Because the first gate dielectric layer 2221 inthe first transistor 410, the second gate dielectric layer 2222 in thesecond transistor 420, and the gate dielectric layer 222 in the thirdtransistor 430 are different only in composition but not in dimensions,the functional metal gate stack 236 is structurally and dimensionallyuniform across the different device regions 1100, 1200, and 1300.

The functional metal gate stack 236 may include one or more workfunction layer and a metal fill layer. N-type devices and p-type devicesmay have different work functional layers. In some implementations,while n-type device regions and p-type device regions may share certaincommon work function layers, n-type device regions may include one ormore work function layers that are not present in the p-type deviceregions. Similarly, in alternative implementations, p-type deviceregions may include one or more work function layers that are notpresent in the n-type device regions. P-type work function layerincludes any suitable p-type work function material, such as TiN, TaN,TaSN, Ru, Mo, Al, WN, WCN ZrSi₂, MoSi₂, TaSi₂, NiSi₂, other p-type workfunction material, or combinations thereof. N-type work function layerincludes any suitable n-type work function material, such as Ti, Al, Ag,Mn, Zr, TiAl, TiAlC, TiAlSiC, TaC, TaCN, TaSiN, TaAl, TaAlC, TaSiAlC,TiAlN, other n-type work function material, or combinations thereof. Itis noted that p-type work function layers are not limited to use inp-type device regions and n-type work function layers are not limited touse in n-type device regions. P-type work function layers and n-typework function layers may be applied in n-type device regions and p-typedevice regions to achieve desired threshold voltage. The metal filllayer may be deposited on n-type work function layer(s) and p-type workfunction layer(s), such that metal fill layer fills any remainingportion of gate trenches in different device regions. The metal filllayer may include a suitable conductive material, such as aluminum (Al),tungsten (W), ruthenium (Ru), and/or copper (Cu). The metal fill layermay additionally or collectively include other metals, metal nitrides,other suitable materials, or combinations thereof.

The first transistor 410 in the first device region 1100, the secondtransistor 420 in the second device region 1200, and the thirdtransistor 430 in the third device region have different thresholdvoltages due to different interface dipoles as a result of use of thefirst gate dielectric layer 2221, the second gate dielectric layer 2222,and the gate dielectric layer 222. Different metal oxides have differentareal density of oxygen atoms. For example, areal densities of oxygenatoms in lanthanum oxide and yttrium oxide are greater than that ofsilicon oxide while areal densities of oxygen atoms in aluminum oxideand hafnium oxide are greater than that of the silicon oxide. Out ofthese metal oxides, aluminum oxide has the highest areal density ofoxygen atoms. In embodiments where the gate dielectric layer 222consists essentially of hafnium oxide and the interfacial layer 220consists essentially of silicon oxide, an interface dipole toward thegate dielectric layer 222 may be formed. When lanthanum or yttrium(having lower areal densities of oxygen atoms than silicon oxide) in thefirst dipole layer 224 and the second dipole layer 230 is allowed todiffuse into the gate dielectric layer 222, the interface dipole towardthe gate dielectric layer may be reduced or reversed. When aluminum(having higher areal densities of oxygen atoms than silicon oxide) inthe first dipole layer 224 and the second dipole layer 230 is allowed todiffuse into the gate dielectric layer 222, the interface dipole towardthe gate dielectric layer may be increased.

In one embodiment where the first gate dielectric layer 2221 has thefirst lanthanum concentration (i.e., first lanthanum to hafnium ratio)at about 0.4 ([La]/[Hf]), the second gate dielectric layer 2222 has thesecond lanthanum concentration (i.e., second lanthanum to hafnium ratio)at about 0.2 ([La]/[Hf]), and the gate dielectric layer 222 has the zerothird lanthanum concentration (i.e., third lanthanum to hafnium ratio)at about 0 ([La]/[Hf]), the first transistor 410 has a first thresholdvoltage (Vt1), the second transistor 420 has a second threshold voltage(Vt2) and the third transistor 430 has a third threshold voltage (Vt3).When the first transistor 410, the second transistor 420, and the thirdtransistor 430 are n-type transistors, Vt1 may be lower than Vt3 byabout 150 mV and Vt2 may be lower than Vt3 by about 50 mV. When thefirst transistor 410, the second transistor 420 and the third transistor430 are p-type transistors, Vt1 may be greater than Vt3 by about 150 mVand Vt1 may be greater than Vt2 by about 50 mV.

The first transistor 410, the second transistor 420, and the thirdtransistor 430 may be implemented in a static random access memory(SRAM) cell to improve its signal-to-noise margin (SNM) and write margin(WM). For example, the first transistor 410, the second transistor 420,and the third transistor 430 may be implemented in an eight-transistor(8T) SRAM cell 900 shown in FIGS. 29 and 30 or a ten-transistor (10T)SRAM cell 1000 shown in FIGS. 31 and 32 .

Reference is first made to a circuit diagram of the 8T SRAM cell 900shown in FIG. 29 . The 8T SRAM cell 900 includes a first pull-uptransistor (PU1) 906 and a first pull-down transistor (PD1) 910 forminga first inverter, a second pull-up transistor (PU2) 908 and a secondpull-down transistor (PD2) 912 forming a second inverter cross-coupledwith the first inverter, a first pass-gate transistor (PG1) 902 and asecond pass-gate transistor (PG2) 904 configured to write data to bestored by the cross-coupled first and second inverters. The 8T SRAM cell900 further includes a read pull-down transistor (RPD) 914 and a readpass-gate transistor (RPG) 916 forming a read port (RP) 918 to accessdata stored by the cross-coupled first and second inverters. Drainelectrodes of the first pull-up transistor (PU1) 906, the firstpull-down transistor (PD1) 910, and the first pass-gate transistor (PG1)902 are electrically connected at a first data storage node Q. Drainelectrodes of the second pull-up transistor (PU2) 908, the secondpull-down transistor (PD2) 912, and the second pass-gate transistor(PG2) 904 are electrically connected at a second data storage node QB.Gate electrodes of the second pull-up transistor (PU2) 908 and thesecond pull-down transistor (PD2) 912 are electrically connected to thedrain electrodes of the first pull-down transistor (PD1) 910, the firstpass-gate transistor (PG1) 902, and the first pull-up transistor (PU1)906 through the first data storage node Q, while gate electrodes of thefirst pull-up transistor (PU1) 906 and the first pull-down transistor(PD1) 910 are electrically connected to the drain electrodes of thesecond pull-down transistor (PD2) 912, the second pass-gate transistor(PG2) 904, and the second pull-up transistor (PU2) 908 through thesecond data storage node QB. Source electrodes of the first pull-downtransistor (PD1) 910, second pull-down transistors (PD2) 912 and theread pull-down transistor (RPD) are connected to a first power supplynode Vss, while source electrodes of the first and second pull-uptransistors (PU1) 906 and (PU2) 908 are connected to a second powersupply node Vdd. According to one embodiment, the first power supplynode Vss is electrically connected to a virtual ground 920, and thesecond power supply node Vdd is electrically connected to a positiveelectrical potential, supplied from a power supply circuit (not shown)of the SRAM. In some implementations, the virtual ground 920 includes ahold transistor 922 and a power gate transistor 924.

The circuit diagram in FIG. 29 may be implemented in a layout of the 8TSRAM cell 900 shown in FIG. 30 . In some implementations, each of thefirst pass-gate transistor (PG1) 902, the second pass-gate transistor(PG2) 904, the first pull-down transistor (PD1) 910, the secondpull-down transistor (PD2) 912, the read pull-down transistor (RPD) 914,and the read pass-gate transistor (RPG) 916 includes a gate structure903 disposed over two active regions 905. Each of the first pull-uptransistor (PU1) 906 and the second pull-up transistor (PU2) 908includes a gate structure 903 (including a first gate structure 903-1and a second gate structure 903-2) disposed over a single active region905. In instances where the active regions are fin-shaped, the formermay be referred to as double-fin devices and the latter may be referredto as single-fin devices.

In an embodiment, the power gate transistor 924 may be a firsttransistor 410. As the first transistor 410 has the highest thresholdvoltage (the first threshold voltage, Vt1) among the first transistor410, the second transistor 420, and the third transistor 430, it has thesmallest leakage current and may reduce leakage at the power gatetransistor 924. In the same embodiment, the read port (RP) 918 may beformed of the third transistor 430. As the third transistor 430 has thelowest threshold voltage (the third threshold voltage, Vt3) among thefirst transistor 410, the second transistor 420, and the thirdtransistor 430, it has the highest switching speed and may improve readspeed of the 8T SRAM cell 900. In the same embodiment, the secondtransistor 420, which has the median threshold voltage level (the secondthreshold voltage Vt2), may be applied to each of the first pass-gatetransistor (PG1) 902, the second pass-gate transistor (PG2) 904, thefirst pull-down transistor (PD1) 910, and the second pull-downtransistor (PD2) 912 to have balanced SNMs. As the first gate structure903-1 is shared among the first pull-down transistor (PD1) 910, thesecond pass-gate transistor (PG2) 904, and the read pass-gate transistor(RPG) 916, the first gate structure 903-1 is shared among two secondtransistors 420 (implemented as PD1 and PG1) and one third transistor430 (implemented as RPG). Similarly, as the second gate structure 903-2is shared among the first pass-gate transistor (PG1) 902, the secondpull-down transistor (PD2) 912, and the read pull-down transistor (RPD)914, the second gate structure 903-2 is shared among two secondtransistors 420 (implemented as PD2 and PG2) and one third transistor430 (implemented as RPD). As used herein, a gate structure is referredto as being shared by or multiple transistors as its functional metalgate stack extend over these transistors. Reference is made to FIG. 15 .While the first transistor 410, the second transistor 420, and the thirdtransistor 430 have different gate dielectric layers, they may have acommon functional metal gate stack 236. The common functional metal gatestack 236 allows transistors of the present disclosure to be sharedamong more than one transistor.

Reference is then made to a circuit diagram of the 10T SRAM cell 1000shown in FIG. 31 . The 10T SRAM cell 1000 includes a first pull-uptransistor (PU1) 1006 and a first pull-down transistor (PD1) 1010forming a first inverter, a second pull-up transistor (PU2) 1008 and asecond pull-down transistor (PD2) 1012 forming a second invertercross-coupled with the first inverter, a first pass-gate transistor(PG1) 1002 and a second pass-gate transistor (PG2) 1004 configured towrite data to be stored by the cross-coupled first and second inverters.The 10T SRAM cell 1000 further includes a first read port (RP1) 1014 anda second read port (RP) 1016 to access data stored by the cross-coupledfirst and second inverters. The first read port (RP) 1014 includes afirst read pull-down transistor (RPD1) 1018 and a first read pass-gatetransistor (RPG1) 1020 and the second read port (RP2) 1016 includes asecond read pull-down transistor (RPD2) 1022 and a second read pass-gatetransistor (RPG2) 1024. Drain electrodes of the first pull-up transistor(PU1) 1006, the first pull-down transistor (PD1) 1010, and the firstpass-gate transistor (PG1) 1002 are electrically connected at a firstdata storage node Q. Drain electrodes of the second pull-up transistor(PU2) 1008, the second pull-down transistor (PD2) 1012, and the secondpass-gate transistor (PG2) 1004 are electrically connected at a seconddata storage node QB. Gate electrodes of the second pull-up transistor(PU2) 1008 and the second pull-down transistor (PD2) 1012 areelectrically connected to the drain electrodes of the first pull-downtransistor (PD1) 1010, the first pass-gate transistor (PG1) 1002, andthe first pull-up transistor (PU1) 1006 through the first data storagenode Q, while gate electrodes of the first pull-up transistor (PU1) 1006and the first pull-down transistor (PD1) 1010 are electrically connectedto the drain electrodes of the second pull-down transistor (PD2) 1012,the second pass-gate transistor (PG2) 1004, and the second pull-uptransistor (PU2) 1008 through the second data storage node QB. Sourceelectrodes of the first pull-down transistor (PD1) 1010, the secondpull-down transistor (PD2) 1012, the first read pull-down transistor(RPD1) 1018, and the second read pull-down transistor (RPD2) 1022 areconnected to a first power supply node Vss, while source electrodes ofthe first and second pull-up transistors (PU1) 1006 and (PU2) 1008 areconnected to a second power supply node Vdd. According to oneembodiment, the first power supply node Vss is electrically connected toa virtual ground 1026, and the second power supply node Vdd iselectrically connected to a positive electrical potential, supplied froma power supply circuit (not shown) of the SRAM. In some implementations,the virtual ground 1026 includes a hold transistor 1028 and a power gatetransistor 1030.

The circuit diagram in FIG. 31 may be implemented in a layout of the 10TSRAM cell 1000 shown in FIG. 32 . In some implementations, each of thefirst pass-gate transistor (PG1) 1002, the second pass-gate transistor(PG2) 1004, the first pull-down transistor (PD1) 1010, the secondpull-down transistor (PD2) 1012, the first read pull-down transistor(RPD1) 1018, the first read pass-gate transistor (RPG1) 1020, the secondread pull-down transistor (RPD2) 1022, the second read pass-gatetransistor (RPG2) 1024 includes a gate structure 1003 disposed over twoactive regions 1005. Each of the first pull-up transistor (PU1) 1006 andthe second pull-up transistor (PU2) 1008 includes a gate structure 1003(including a first gate structure 1003-1 and a second gate structure1003-2) disposed over a single active region 1005. In instances wherethe active regions are fin-shaped, the former may be referred to asdouble-fin devices and the latter may be referred to as single-findevices.

In an embodiment, the power gate transistor 1030 may be a firsttransistor 410. As the first transistor 410 has the highest thresholdvoltage (the first threshold voltage, Vt1) among the first transistor410, the second transistor 420, and the third transistor 430, it has thesmallest leakage current and may reduce leakage at the power gatetransistor 1030. In the same embodiment, the first read port (RP1) 1014and the second read port (RP2) 1016 may be formed of the thirdtransistor 430. As the third transistor 430 has the lowest thresholdvoltage (the third threshold voltage, Vt3) among the first transistor410, the second transistor 420, and the third transistor 430, it has thehighest switching speed and may improve read speed of the 10T SRAM cell1000. In the same embodiment, the second transistor 420, which has themedian threshold voltage level (the second threshold voltage Vt2), maybe applied to each of the first pass-gate transistor (PG1) 1002, thesecond pass-gate transistor (PG-2) 1004, the first pull-down transistor(PD1) 1010, and the second pull-down transistor (PD2) 1012 to havebalanced SNMs. As the first gate structure 1003-1 is shared among firstread pull-down transistor (RPD1) 1018, the first pull-down transistor(PD1) 1010, the second pass-gate transistor (PG2) 1004, and the secondread pass-gate transistor (RPG2) 1024, the first gate structure 1003-1is shared among two second transistors 420 (implemented as PD1 and PG1)and two third transistors 430 (implemented as RPD1 and RPG2). Similarly,as the second gate structure 1003-2 is shared among first read pass-gatetransistor (RPG1) 1020, the first pass-gate transistor (PG1) 1002, thesecond pull-down transistor (PD2) 1012, and the second read pull-downtransistor (RPD2) 1022, the second gate structure 1003-2 is shared amongtwo second transistors 420 (implemented as PD2 and PG2) and two thirdtransistor 430 (implemented as RPG1 and RPD2). As described above, agate structure is referred to as being shared by or multiple transistorsas its functional metal gate stack extend over these transistors.Reference is made to FIGS. 28A and 28B. While the first transistor 810,the second transistor 820, the third transistor 830, and the fourthtransistor 840 have different gate dielectric layers, they may have acommon functional metal gate stack 236. The common functional metal gatestack 236 allows transistors of the present disclosure to be sharedamong more than one transistor.

FIG. 16 illustrates a flow chart of a method 500 for forming asemiconductor device according to various aspects of the presentdisclosure. FIGS. 2-5, 17-27, 28A and 28B are fragmentarycross-sectional views of a workpiece 200 at various stages offabrication of the method 500 in FIG. 16 . Additional steps can beprovided before, during, and after method 500, and some of the stepsdescribed can be moved, replaced, or eliminated for additionalembodiments of method 500. Additional features can be added in thecontact structure depicted in FIGS. 2-5, 17-27, 28A and 28B, and some ofthe features described below can be replaced, modified, or eliminated inother embodiments of the interconnect structure depicted in FIGS. 2-5,17-27, 28A and 28B.

Referring to FIGS. 16 and 2 , the method 500 includes a block 502 wherea workpiece 200 is received. As operations at block 502 are similar tothose at block 102 described above, descriptions thereof will not berepeated. In addition, detailed descriptions of the workpiece 200 andvarious features thereon have been described above and are also omittedhere for brevity.

Referring to FIGS. 16 and 3 , the method 500 includes a block 504 wherethe dummy gate stack 208 is removed to form a gate trench 218. Asoperations at block 504 are similar to those at block 104 describedabove, descriptions thereof will not be repeated. In addition, detaileddescriptions of the dummy gate stack 208 and the gate trench 218 havebeen described above and are also omitted here for brevity.

Referring to FIGS. 16 and 4 , the method 500 includes a block 506 wherean interfacial layer 220 is deposited in the gate trench 218. Asoperations at block 506 are similar to those at block 106 describedabove, descriptions thereof will not be repeated. In addition, detaileddescriptions of the interfacial layer 220 have been described above andare also omitted here for brevity.

Referring to FIGS. 16 and 5 , the method 500 includes a block 508 wherea gate dielectric layer 222 is deposited over the interfacial layer 220.As operations at block 508 are similar to those at block 108 describedabove, descriptions thereof will not be repeated. In addition, detaileddescriptions of the gate dielectric layer 222 have been described aboveand are also omitted here for brevity.

As will be described below, the workpiece 200 may include multipledevice regions (such as three device regions, four device regions, ormore device regions) for transistors having different threshold voltagesand the structure shown in FIGS. 2-5 may be repeated across thesemultiple device regions but the repeated structures are omitted, whichis representatively shown by ellipses (“ . . . ”) in FIGS. 2-5 .Operations of the method 500 that treat different device regionsdifferently will be described below in conjunction with FIGS. 17-27, 28Aand 28B. For simplicity and clarity of descriptions, FIGS. 17-27illustrate fragmentary cross-sectional views of area “I” in differentdevice regions.

Referring to FIGS. 16 and 6 , the method 500 includes a block 510 wherea first dipole layer 224 is deposited over the gate dielectric layer222. In the embodiments illustrated in FIGS. 17-27, 28A and 28B, theworkpiece 200 includes four device regions—a first device region 3100, asecond device region 3200, a third device region 3300, and a fourthdevice region 3400. As described above, FIG. 17 illustrates thefragmentary cross-sectional views of the area “I” in the first deviceregion 3100, the second device region 3200, the third device region3300, and the fourth device region 3400. The first dipole layer 224 isdeposited on the gate dielectric layer 222 in the gate trenches in thefirst device region 3100, the second device region 3200, the thirddevice region 3300, and the fourth device region 3400. In someembodiments, the first dipole layer 224 may be formed of lanthanumoxide, yttrium oxide, or aluminum oxide and may be deposited usingatomic layer deposition (ALD). In some implementations, the ALD processused to form the first dipole layer 224 may include between about 2 andabout 10 cycles. In those implementations, the first dipole layer 224may have a thickness between about 1 Å and about 10 Å. In oneembodiment, the first dipole layer 224 may be formed of lanthanum oxide.

Referring to FIGS. 16, 18, 19, and 20 , the method 500 includes a block512 where the first dipole layer 224 is selectively removed from thethird device region 3300 and the fourth device region 3400. In someembodiments, photolithography techniques and etch techniques may be usedto perform the operations at block 512. An example process is shown inFIGS. 18-20 . Reference is first made to FIG. 18 . A hard mask layer 226is first formed over the first dipole layer 224 and a bottomantireflective coating (BARC) layer 228 is deposited over the hard masklayer 226. In some instances, the hard mask layer 226 may be a singlelayer or a multi-layer. When the hard mask layer 226 is a single layer,the hard mask layer 226 may include silicon oxide, silicon nitride, orsilicon oxynitride. When the hard mask layer 226 is a multi-layer, thehard mask layer 226 may include a silicon layer and a silicon nitridelayer on the silicon layer. The BARC layer 228 may include siliconoxynitride, a polymer, or a suitable material. To pattern the BARC layer228 and the hard mask layer 226, a photoresist layer may be blanketlydeposited over the workpiece 200, including over the BARC layer 228 inthe first device region 3100, the second device region 3200, the thirddevice region 3300, and the fourth device region 3400. The photoresistlayer may be a single layer or a multi-layer, such as a tri-layer. Thephotoresist layer is then exposed to radiation going through orreflected from a mask, baked in a post-bake process, and developed in adeveloper solution to form a patterned photoresist mask. The BARC layer228 and the hard mask layer 226 are then patterned using the patternedphotoresist mask to form an etch mask having openings over the thirddevice region 3300 and the fourth device region 3400. The etch mask isthen used in an etch process to selectively etch away the first dipolelayer 224 in the gate trench in the third device region 3300 and thefourth device region 3400, as illustrated in FIG. 19 . The etch processmay be a dry etch process, a wet etch process, or a suitable etchprocess. Referring to FIG. 20 , after the first dipole layer 224 isselectively removed from the gate trench in the third device region 3300and the fourth device region 3400, the hard mask layer 226 and the BARClayer 228 in the first device region 3100 and the second device region3200 are removed using a suitable etching process.

Referring to FIGS. 16 and 21 , the method 500 includes a block 514 wherea first anneal process 600 is performed to anneal the workpiece 200. Atblock 514, the first anneal process 600 is used to thermally driveelements in the first dipole layer 224 into the gate dielectric layer222 in the gate trenches in the first device region 3100 and the seconddevice region 3200. The first dipole layer 224 serves as a diffusiondoping vehicle to bring its elements to be in direct contact with thegate dielectric layer 222 in the first device region 3100 and the seconddevice region 3200. The first anneal process 600 may be a rapid thermalanneal (RTA) process, a laser spike anneal process, a flash annealprocess, or a furnace anneal process. In some implementation, the firstanneal process 600 includes a high anneal temperature between about 500°C. and about 900° C. so as to allow lanthanum, yttrium, or aluminum inthe first dipole layer 224 to diffuse into the gate dielectric layer 222in gate trenches in the first device region 3100 and the second deviceregion 3200. Because the gate trenches in the third device region 3300and the fourth device region 3400 are free of the first dipole layer224, the first anneal process 600 at block 514 does not result in anydipole layer material diffusing into the gate dielectric layer 222 inthe third device region 3300 and the fourth device region 3400. In someimplementations, the first anneal process 600 may last between about 5seconds and about 20 seconds. As shown in FIG. 22 , after the firstanneal process 600 at block 514, elements in the first dipole layer 224diffuse into the gate dielectric layers 222 to form the second gatedielectric layer 2232 in gate trenches in the first device region 3100and the second device region 3200.

Referring to FIGS. 16 and 22 , the method 500 includes a block 516 wherethe first dipole layer 224 is selectively removed from the workpiece200. In some embodiments, photolithography and etch techniques may beused at block 516 to prevent damages to the gate dielectric layer 222 inthe third device region 3300 and the fourth device region 3400. Forexample, a hard mask layer, a BARC layer, and a photoresist layer may bedeposited over the workpiece 200. The photoresist layer, the BARC layer,and the hard mask layer are then patterned to form an etch mask thatexpose the first dipole layer 224 in the gate trenches in the firstdevice region 3100 and the second device region 3200. The first dipolelayer 224 is then etched by a dry etch process, a wet etch process, or asuitable etch process using the etch mask. The etch mask, which isformed of the hard mask layer and the BARC layer, is then removed. Atthe conclusion of block 516, the second gate dielectric layer 2232 inthe gate trenches in the first device region 3100 and the second deviceregion 3200 and the gate dielectric layer 222 in the gate trenches inthe third device region 3300 and the fourth device region 3400 areexposed.

Referring to FIGS. 16 and 23 , the method 500 includes a block 518 wherea second dipole layer 240 is deposited over the workpiece 200. As shownin FIG. 23 , the second dipole layer 240 is deposited on the second gatedielectric layer 2232 in the gate trenches in the first device region3100 and the second device region 3200 as well as on the gate dielectriclayer 222 in the gate trenches in the third device region 3300 and thefourth device region 3400. In some embodiments, the second dipole layer240 and the first dipole layer 224 may have the same composition.Similar to the first dipole layer 224, the second dipole layer 240 mayalso be formed of lanthanum oxide, yttrium oxide, or aluminum oxide andmay be deposited using atomic layer deposition (ALD). In someimplementations, the ALD process used to form the second dipole layer240 may include between about 2 and about 5 cycles. In thoseimplementations, the second dipole layer 240 may have a thicknessbetween about 1 Å and about 5 Å. In one embodiment, the second dipolelayer 240 may be formed of lanthanum oxide. Different from the seconddipole layer 230 in the embodiments illustrated in FIGS. 2-15 , thesecond dipole layer 240 in the embodiments illustrated in FIGS. 17-27,28A and 28B are thinner. The second dipole layer 240, with a thicknessbetween about 1 Å and about 5 Å, is thinner than the first dipole layer224, which has a thickness between about 1 Å and about 10 Å.

Referring to FIGS. 16, 24, 25, and 26 , the method 500 includes a block520 where the second dipole layer 240 is selectively removed from thegate trenches in the second device region 3200 and the fourth deviceregion 3400. Similar to operations at block 112 in method 100,operations at block 520 may also be performed using photolithography andetch techniques. For example, a hard mask layer 242 and a BARC layer 244may be formed over the second dipole layer 240, as shown in FIG. 24 . Asthe hard mask layer 242 may be similar to the hard mask layer 226 andthe BARC layer 244 may be similar to the BARC layer 228, detaileddescriptions of the hard mask layer 242 and the BARC layer 244 areomitted for brevity. Thereafter a photoresist layer may then bedeposited over the BARC layer 244. The photoresist layer, the BARC layer244, and the hard mask layer 242 are then patterned in fashions similarto those described with respect to block 112 of method 100 and will notbe repeated here. The patterned hard mask layer 242 allows selectiveremoval of the second dipole layer 240 in the gate trenches in thesecond device region 3200 and the fourth device region 3400, exposingthe second gate dielectric layer 2232 in the second device region 3200and gate dielectric layer 222 in the fourth device region 3400. Asillustrated in FIG. 25 , at this point, the gate trench in the firstdevice region 3100 includes thereover the second gate dielectric layer2232 and the second dipole layer 240; the gate trench in the seconddevice region 3200 includes thereover the second gate dielectric layer2232; the gate trench in the third device region 3300 includes thereoverthe second dipole layer 240; and the gate trench in the fourth deviceregion 3400 includes the gate dielectric layer 222 uncovered by anydipole layer. After the second dipole layer 240 is selectively removedfrom the gate trenches in the second device region 3200 and the fourthdevice region 3400, the hard mask layer 242 and the BARC layer 244 maythen be removed from the first device region 3100 and the third deviceregion 3300, as illustrated in FIG. 26 .

Referring to FIGS. 16 and 26 , the method 500 includes a block 522 wherethe workpiece 200 is annealed in a second anneal process 700. At block522, the second anneal process 700 is used to thermally drive elementsin the second dipole layer 240 into the second gate dielectric layer2232 in the gate trench in the first device region 3100 and into thegate dielectric layer 222 in the gate trench in the third device region3300. The second dipole layer 240 serves as a diffusion doping vehicleto bring its elements to be in direct contact with the second gatedielectric layer 2232 and the gate dielectric layer 222. The secondanneal process 700 may be a rapid thermal anneal (RTA) process, a laserspike anneal process, a flash anneal process, or a furnace annealprocess. In some implementation, the second anneal process 700 includesa high anneal temperature between about 500° C. and about 900° C. so asto allow lanthanum, yttrium, or aluminum in the second dipole layer 240to diffuse into the second gate dielectric layer 2232 in gate trench inthe first device region 3100 and into the gate dielectric layer 222 inthe gate trench in the third device region 3300. Because the gate trenchin the fourth device region 3400 is free of any dipole layer, the secondanneal process 700 at block 522 does not result in any dipole layermaterial diffusing into the gate dielectric layer 222 in the fourthdevice region 3400. In some implementations, the second anneal process700 may last between about 5 seconds and about 20 seconds.

Referring to FIGS. 16 and 27 , the method 500 include a block 524 wherethe second dipole layer 240 are removed from the workpiece 200. Theoperations at block 524 may be performed using a dry etch process, a wetetch process, or a suitable etch process. After the element in thesecond dipole layer 240 is thermally driven into the second gatedielectric layer 2232 in the first device region 3100 and gatedielectric layer 222 in the third device region 3300. At block 524, thesecond dipole layer 240 is removed from the gate trenches in the firstdevice region 3100 and the third device region 3300. Due to the secondanneal process 700 at block 522, a first gate dielectric layer 2231 isformed over the gate trench in the first device region 3100 and a thirdgate dielectric layer 2233 is formed over the gate trench in the thirddevice region 3300. To summarize, the first gate dielectric layer 2231in the first device region 3100 is formed as a result of the firstdipole layer 224 driven in by the first anneal process 600 and thesecond dipole layer 240 driven in by the second anneal process 700; thesecond gate dielectric layer 2232 in the second device region 3200 isformed as a result of the first dipole layer 224 driven in by the firstanneal process 600; the third gate dielectric layer 2233 in the thirddevice region 3300 is formed as a result of the second dipole layer 240driven in by the second anneal process 700, and the gate dielectriclayer 222 in the fourth device region 3400 is free of diffusion elementsfrom any dipole layer.

It has also been observed that a thicker dipole layer and increasedanneal processes contribute to a greater doping concentration of thedipole layer material in the gate dielectric layer 222. For example,when the gate dielectric layer 222 is formed of hafnium oxide and thefirst dipole layer 224/second dipole layer 240 is formed of lanthanumoxide, operations at block 626 may result in a first lanthanumconcentration in the first gate dielectric layer 2231 in the firstdevice region 3100, a second lanthanum concentration in the second gatedielectric layer 2232 in the second device region 3200, a thirdlanthanum concentration in the third device region 3300, and a fourthlanthanum concentration in the fourth device region 3400. The firstlanthanum concentration is greater than the second lanthanumconcentration, the second lanthanum concentration is greater than thethird lanthanum concentration, and the third lanthanum concentration isgreater than the fourth lanthanum concentration. Because the gate trenchin the fourth device region 3400 is free of any dipole layer, the fourthlanthanum concentration that is zero. Each of the first, second, third,and fourth lanthanum concentrations may be represented as a ratio oflanthanum concentration (i.e., [lathanum] or [La] from the first dipolelayer 224/second dipole layer 240) to hafnium (i.e., [Hafnium] or [Hf]in the gate dielectric layer 222). In the example described above, thefirst lanthanum concentration (i.e., first lanthanum to hafnium ratio)may be about 0.6 ([La]/[Hf]), the second lanthanum concentration (i.e.,second lanthanum to hafnium ratio) may be about 0.4 ([La]/[Hf]), and thethird lanthanum concentration (i.e., third lanthanum to hafnium ratio)may be about 0.2 ([La]/[Hf]), while the fourth lanthanum concentration(i.e., fourth lanthanum to hafnium ratio) is zero. The foregoingdescription generally applies to other dipole layer materials, such asyttrium and aluminum, and other gate dielectric material, such as HfSiO,HfSiON, HfTaO, HfTiO, HfZrO, ZrO₂, TiO₂, Ta₂O₅, provided that differentdipole layer material may have different diffusivity and differentdipole layer may have different solid solubility in different gatedielectric layers.

Referring to FIGS. 16, 28A and 28B, the method 500 includes a block 526where further processes are performed to form a first transistor 810 inthe first device region 3100, a second transistor 820 in the seconddevice region 3200, a third transistor 830 in the third device region3300, and a fourth transistor 840 in the fourth device region 3400. Asshown in FIGS. 28A and 28B, the first transistor 810 includes the firstgate dielectric layer 2231, the second transistor 820 includes thesecond gate dielectric layer 2232, the third transistor 830 includes thethird gate dielectric layer 2233, and the fourth transistor 840 includesthe gate dielectric layer 222. Each of the first transistor 810, thesecond transistor 820, the third transistor 830, and the fourthtransistor 840 includes a functional metal gate stack 236 in addition tothe interfacial layer 220 and the first gate dielectric layer2231/second gate dielectric layer 2232/third gate dielectric layer2233/gate dielectric layer 222. Because the first gate dielectric layer2231 in the first transistor 810, the second gate dielectric layer 2232in the second transistor 820, the third gate dielectric layer 2233 inthe third transistor 830, and the gate dielectric layer 222 in thefourth transistor 840 are different only in composition but not indimensions, the functional metal gate stack 236 is structurally anddimensionally uniform across the different device regions 3100, 3200,3300, and 3400.

The functional metal gate stack 236 may include one or more workfunction layer and a metal fill layer. N-type devices and p-type devicesmay have different work functional layers. In some implementations,while n-type device regions and p-type device regions may share certaincommon work function layers, n-type device regions may include one ormore work function layers that are not present in the p-type deviceregions. Similarly, in alternative implementations, p-type deviceregions may include one or more work function layers that are notpresent in the n-type device regions. P-type work function layerincludes any suitable p-type work function material, such as TiN, TaN,TaSN, Ru, Mo, Al, WN, WCN ZrSi₂, MoSi₂, TaSi₂, NiSi₂, other p-type workfunction material, or combinations thereof. N-type work function layerincludes any suitable n-type work function material, such as Ti, Al, Ag,Mn, Zr, TiAl, TiAlC, TiAlSiC, TaC, TaCN, TaSiN, TaAl, TaAlC, TaSiAlC,TiAlN, other n-type work function material, or combinations thereof. Itis noted that p-type work function layers are not limited to use inp-type device regions and n-type work function layers are not limited touse in n-type device regions. P-type work function layers and n-typework function layers may be applied in n-type device regions and p-typedevice regions to achieve desired threshold voltage. The metal filllayer may be deposited on n-type work function layer(s) and p-type workfunction layer(s), such that metal fill layer fills any remainingportion of gate trenches in different device regions. The metal filllayer may include a suitable conductive material, such as aluminum (Al),tungsten (W), ruthenium (Ru), and/or copper (Cu). The metal fill layermay additionally or collectively include other metals, metal nitrides,other suitable materials, or combinations thereof.

The first transistor 810 in the first device region 3100, the secondtransistor 820 in the second device region 3200, the third transistor830 in the third device region 3300, and the fourth transistor 840 inthe fourth device region 3400 have different threshold voltages due todifferent interface dipoles as a result of use of the first gatedielectric layer 2231, the second gate dielectric layer 2232, the thirdgate dielectric layer 2233, and the gate dielectric layer 222. Differentmetal oxides have different areal density of oxygen atoms. For example,areal densities of oxygen atoms in lanthanum oxide and yttrium oxide aregreater than that of silicon oxide while areal densities of oxygen atomsin aluminum oxide and hafnium oxide are greater than that of the siliconoxide. Out of these metal oxides, aluminum oxide has the highest arealdensity of oxygen atoms. In embodiments where the gate dielectric layer222 consists essentially of hafnium oxide and the interfacial layer 220consists essentially of silicon oxide, an interface dipole toward thegate dielectric layer 222 may be formed. When lanthanum or yttrium(having lower areal densities of oxygen atoms than silicon oxide) in thefirst dipole layer 224 and the second dipole layer 240 is allowed todiffuse into the gate dielectric layer 222, the interface dipole towardthe gate dielectric layer may be reduced or reversed. When aluminum(having higher areal densities of oxygen atoms than silicon oxide) inthe first dipole layer 224 and the second dipole layer 240 is allowed todiffuse into the gate dielectric layer 222, the interface dipole towardthe gate dielectric layer may be increased.

In one embodiment where the first gate dielectric layer 2231 has thefirst lanthanum concentration (i.e., first lanthanum to hafnium ratio)at about 0.6 ([La]/[Hf]), the second gate dielectric layer 2232 has thesecond lanthanum concentration (i.e., second lanthanum to hafnium ratio)at about 0.4 ([La]/[Hf]), the third gate dielectric layer 2233 has thethird lanthanum concentration (i.e., third lanthanum to hafnium ratio)at about 0.2 ([La]/[Hf]), and the gate dielectric layer 222 has the zerofourth lanthanum concentration (i.e., fourth lanthanum to hafnium ratio)at about 0 ([La]/[Hf]), the first transistor 810 has a first thresholdvoltage (Vt1), the second transistor 820 has a second threshold voltage(Vt2), the third transistor 830 has a third threshold voltage (Vt3), andthe fourth transistor 840 has a fourth threshold voltage (Vt4). When thefirst transistor 810, the second transistor 820, the third transistor830, and the fourth transistor 840 are n-type transistors, Vt1 may belower than Vt4 by about 250 mV, Vt2 may be lower than Vt4 by about 150mV, and Vt3 may be lower than Vt4 by about 50 mV. When the firsttransistor 810, the second transistor 820, the third transistor 830, andthe fourth transistor 840 are p-type transistors, Vt1 may be greaterthan Vt4 by about 250 mV, Vt1 may be greater than Vt3 by about 150 mV,and Vt1 may be greater than Vt2 by about 50 mV.

The first transistor 810, the second transistor 820, the thirdtransistor 830, and the fourth transistor 840 may be implemented in in astatic random access memory (SRAM) cell to improve its signal-to-noisemargin (SNM) and write margin (WM). For example, the first transistor810, the second transistor 820, the third transistor 830, and the fourthtransistor 840 may be implemented in the 8T SRAM cell 900 shown in FIGS.29 and 30 or the 10T SRAM cell 1000 shown in FIGS. 31 and 32 .

In some embodiments, the first transistor 810, the second transistor820, the third transistor 830, and the fourth transistor 840 may beimplemented in the 8T SRAM cell 900 shown in FIGS. 29 and 30 . In oneexample, the first transistor 810, with the first threshold voltage Vt1,may be implemented as the first pass-gate transistor (PG1) 902 and thesecond pass-gate transistor (PG2) 904; the second transistor 820, withthe second threshold voltage Vt2, may be implemented as the firstpull-down transistor (PD1) 910 and the second pull-down transistor (PD2)912; the third transistor 830, with the third threshold voltage Vt3, maybe implemented as the read pass-gate transistor (RPG) 916; and thefourth transistor 840, with the fourth threshold voltage Vt4, may beimplemented as the read pull-down transistor (RPD) 914. In this example,because the third transistor 830 and the fourth transistor 840 havelower threshold voltages than the first transistor 810 and the secondtransistor 820, the read port 918 may have faster read speed.Additionally, because transistors with greater threshold voltages havesmaller leakage currents (i.e., drive currents), the leakage currentsthrough the first pass-gate transistor (PG1) 902 and the secondpass-gate transistor (PG2) 904 are smaller than those through the firstpull-down transistor (PD1) 910 and the second pull-down transistor (PD2)912. This arrangement may allow the 8T SRAM cell 900 to have anincreased beta ((3) ratio, which translates into increased readstability.

In these embodiments, as the first gate structure 903-1 is shared amongthe first pull-down transistor (PD1) 910, the second pass-gatetransistor (PG2) 904, and the read pass-gate transistor (RPG) 916, thefirst gate structure 903-1 is shared among one first transistor 810(implemented as PG2), one second transistor 820 (implemented as PD1),and one third transistor 830 (implemented as RPG). Similarly, as thesecond gate structure 903-2 is shared among the first pass-gatetransistor (PG1) 902, the second pull-down transistor (PD2) 912, and theread pull-down transistor (RPD) 914, the second gate structure 903-2 isshared among one first transistor 810 (implemented as PG1), one secondtransistor 820 (implemented as PD2) and one fourth transistor 840(implemented as RPD). As described above, a gate structure is referredto as being shared by or multiple transistors as its functional metalgate stack extend over these transistors. Reference is made to FIGS. 28Aand 28B. While the first transistor 810, the second transistor 820, thethird transistor 830, and the fourth transistor 840 have different gatedielectric layers, they may have a common functional metal gate stack236. The common functional metal gate stack 236 allows transistors ofthe present disclosure to be shared among more than one transistor.

In some other embodiments, the first transistor 810, the secondtransistor 820, the third transistor 830, and the fourth transistor 840may be implemented in the 10T SRAM cell 1000 shown in FIGS. 31 and 32 .In one example, the first transistor 810, with the first thresholdvoltage Vt1, may be implemented as the first pass-gate transistor (PG1)1002 and the second pass-gate transistor (PG2) 1004; the secondtransistor 820, with the second threshold voltage Vt2, may beimplemented as the first pull-down transistor (PD1) 1010 and the secondpull-down transistor (PD2) 1012; the third transistor 830, with thethird threshold voltage Vt3, may be implemented as the first readpass-gate transistor (RPG1) 1020 and the second read pass-gatetransistor (RPG2) 1024; and the fourth transistor 840, with the fourththreshold voltage Vt4, may be implemented as the first read pull-downtransistor (RPD1) 1018 and the second read pull-down transistor (RPD2)1022. In this example, because the third transistor 830 and the fourthtransistor 840 have lower threshold voltages than the first transistor810 and the second transistor 820, the first read port 1014 and thesecond read port 1016 may have faster read speeds. Additionally, becausetransistors with greater threshold voltages have smaller leakagecurrents (i.e., drive currents), the leakage currents through the firstpass-gate transistor (PG1) 1002 and the second pass-gate transistor(PG2) 1004 are smaller than those through the first pull-down transistor(PD1) 1010 and the second pull-down transistor (PD2) 1012. Thisarrangement may allow the 10T SRAM cell 1000 to have an increased beta(β) ratio, which translates into increased read stability.

In these other embodiments, as the first gate structure 1003-1 is sharedamong first read pull-down transistor (RPD1) 1018, the first pull-downtransistor (PD1) 1010, the second pass-gate transistor (PG2) 1004, andthe second read pass-gate transistor (RPG2) 1024, the first gatestructure 1003-1 is shared among one first transistor 810 (implementedas PG2), one second transistor 820 (implemented as PD1), one thirdtransistor 830 (implemented as RPG2), and one fourth transistor 840(implemented as RPD1). Similarly, as the second gate structure 1003-2 isshared among first read pass-gate transistor (RPG1) 1020, the firstpass-gate transistor (PG1) 1002, the second pull-down transistor (PD2)1012, and the second read pull-down transistor (RPD2) 1022, the secondgate structure 1003-2 is shared among one first transistor 810(implemented as PG1), one second transistor 820 (implemented as PD2),one third transistor 830 (implemented as RPG1), and one fourthtransistor 840 (implemented as RPD2). As described above, a gatestructure is referred to as being shared by or multiple transistors asits functional metal gate stack extend over these transistors. Referenceis made to FIGS. 28A and 28B. While the first transistor 810, the secondtransistor 820, the third transistor 830, and the fourth transistor 840have different gate dielectric layers, they may have a common functionalmetal gate stack 236. The common functional metal gate stack 236 allowstransistors of the present disclosure to be shared among more than onetransistor.

Throughout the present disclosure, similar reference numerals may beused for similar features with similar compositions, provided thatmultiple device regions and features thereon may be renumbered fordifferent embodiments. For example, the three device regions in FIG. 6are the first device region 1100, the second device region 1200, and thethird device region 1300 while the fourth device regions in FIG. 17 arethe first device region 3100, the second device region 3200, the thirddevice region 3300, and the fourth device region 3400. The samenumbering convention applies to the gate dielectric layers (e.g., 222,2221, 2222 in FIGS. 15 and 222, 2231, 2232, and 2233 in FIGS. 28A and28B), transistors (e.g., 410, 420, and 430 in FIGS. 15 and 810, 820,830, and 840 in FIGS. 28A and 28B). In addition, while the first dipolelayer 224 may be similar with respect to both methods 100 and 500, thesecond dipole layer 230 in method 100 and the second dipole layer 240 inmethod 500 may not be similar. For that reason, they are referred to bydifferent reference numerals. Furthermore, the interfacial layer 220,the gate dielectric layer 222 (or the first, second, or third gatedielectric layer, as the case may be in different embodiments), and thefunctional metal gate stack 236 may be referred to as a metal gatestructure or a functional metal gate structure in some instances.

Methods according to the present disclosure provide a mechanism toprovide for transistors with different threshold voltages withoutimpacting the process window for gate structure formation. Instead ofintroducing an additional dipole layer that is going to stay in the gatetrench, one or more dipole layers are selectively deposited on a gatedielectric layer and serve as vehicles of diffusion dopants to dope thegate dielectric layer. Depending on the thickness of the dipole layer,the anneal process duration, and the material of the dipole layer, thegate dielectric layer being doped may have different interface dipolesat its interface with the interfacial layer. After the doping process,the use of dipole layer(s) is removed from the gate trench. That is,methods of the present disclosure preserve the up-sides of havingdifferent threshold voltages using a dipole layer without the associateddown-sides. The present disclosure provides embodiments to implementthree levels of threshold voltages in a semiconductor device havingthree device regions and embodiments to implement four levels ofthreshold voltages in a semiconductor device having four device regions.Transistors having different levels of threshold voltages of the presentdisclosure may be applied in SRAM cells, such as 8T SRAM cells or 10TSRAM cells, to improve their performance. After reviewing the presentdisclosure, a person of ordinary skill in the art will appreciate thatmore threshold voltages in more device regions are possible.

The present disclosure provides embodiments of semiconductor devices andmethods of forming the same. In one embodiment, the present disclosureprovides a semiconductor device that includes a first transistor, asecond transistor and a third transistor. The first transistor includesa first active region, a first gate dielectric layer over the firstactive region and including a first concentration of a dipole layermaterial, and a first gate structure disposed over the first gatedielectric layer. The second transistor includes a second active region,a second gate dielectric layer over the second active region andincluding a second concentration of the dipole layer material, and asecond gate structure disposed over the second gate dielectric layer.The third transistor includes a third active region, a third gatedielectric layer over the third active region including a thirdconcentration of the dipole layer material, and a third gate structuredisposed over the third gate dielectric layer. The dipole layer materialincludes lanthanum oxide, aluminum oxide, or yttrium oxide. The firstconcentration is greater than the second concentration and the secondconcentration is greater than the third concentration.

In some implementations, the first gate structure, the second gatestructure and the third gate structure are substantially identical toone another. In some instances, the third concentration is zero. In someimplementations, the first transistor, the second transistor and thethird transistor are n-type transistors. The first transistor includes afirst threshold voltage, the second transistor includes a secondthreshold voltage, and the third transistor includes a third thresholdvoltage. The first threshold voltage is smaller than the secondthreshold voltage and the second threshold voltage is smaller than thethird threshold voltage. In some implementations, the first transistor,the second transistor and the third transistor are p-type transistors.The first transistor includes a first threshold voltage, the secondtransistor includes a second threshold voltage, and the third transistorincludes a third threshold voltage. The first threshold voltage isgreater than the second threshold voltage and the second thresholdvoltage is greater than the third threshold voltage. In some instances,the first gate dielectric layer, the second gate dielectric layer andthe third gate dielectric layer further include hafnium oxide.

In another embodiment, the present disclosure provides a method thatincludes providing a workpiece including a first device region, a seconddevice region and a third device region, forming a first gate trench inthe first device region, a second gate trench in the second deviceregion, and a third gate trench in the third device region, depositing agate dielectric layer in the first gate trench, the second gate trench,and the third gate trench, depositing a first dipole layer over the gatedielectric layer in the first gate trench, the second gate trench, andthe third gate trench, selectively removing the first dipole layer inthe second gate trench, depositing a second dipole layer over the firstdipole layer in the first gate trench, the gate dielectric layer in thesecond gate trench, and the first dipole layer in the third gate trench,selectively removing the first dipole layer and the second dipole layerin the third gate trench, and annealing the workpiece.

In some implementations, the method may further include after theannealing of the workpiece, removing the first dipole layer and thesecond dipole layer from the workpiece. In some instances, the firstdipole layer and the second dipole layer include lanthanum oxide,aluminum oxide, or yttrium oxide. In some implementations, the annealingof the workpiece includes a temperature between about 500° C. and about900° C. In some instances, the annealing of the workpiece includes aduration between about 5 seconds and about 20 seconds. In someimplementations, a thickness of the first dipole layer and a thicknessof the second dipole layer are between about 1 Å and about 10 Å. In someinstances, the selectively removing of the first dipole layer in thesecond gate trench includes depositing a hard mask layer over theworkpiece, depositing a bottom antireflective coating (BARC) layer overthe hard mask layer, patterning the BARC layer and the hard mask layerto expose the second gate trench, and removing the first dipole layer inthe second gate trench while the first gate trench and the third gatetrench are covered by the hard mask layer and the BARC layer. In someinstances, the first dipole layer includes a first thickness, the seconddipole layer includes a second thickness, and the first thickness issubstantially identically to the second thickness.

In yet another embodiment, the present disclosure provides a method thatincludes providing a workpiece having a first device region, a seconddevice region, a third device region, and a fourth device region,forming a first gate trench in the first device region, a second gatetrench in the second device region, a third gate trench in the thirddevice region, a fourth gate trench in the fourth device region,depositing a gate dielectric layer in the first gate trench, the secondgate trench, the third gate trench, and the fourth gate trench,depositing a first dipole layer over the gate dielectric layer in thefirst gate trench, the second gate trench, the third gate trench, andthe fourth gate trench, selectively removing the first dipole layer inthe third gate trench and the fourth gate trench, performing a firstanneal process to the workpiece, removing the first dipole layer in thefirst gate trench and the second gate trench, after the removing of thefirst dipole layer, depositing a second dipole layer over the first gatetrench, the second gate trench, the third gate trench, and the fourthgate trench, selectively removing the second dipole layer in the secondgate trench and the fourth gate trench, performing a second annealprocess to the workpiece, and removing the second dipole layer from theworkpiece.

In some implementations, the first dipole layer includes a firstthickness, the second dipole layer includes a second thickness, and thefirst thickness is greater than the second thickness. In some instances,the first thickness is between about 5 Å and about 10 Å and the secondthickness is between about 1 Å and about 5 Å. In some implementations,the first dipole layer and the second dipole layer include lanthanumoxide, aluminum oxide, or yttrium oxide. In some instances, the firstanneal process and the second anneal process includes a temperaturebetween about 500° C. and about 900° C. In some instances, the firstanneal process and the second anneal process includes a duration betweenabout 5 seconds and about 20 seconds.

The foregoing has outlined features of several embodiments. Thoseskilled in the art should appreciate that they may readily use thepresent disclosure as a basis for designing or modifying other processesand structures for carrying out the same purposes and/or achieving thesame advantages of the embodiments introduced herein. Those skilled inthe art should also realize that such equivalent constructions do notdepart from the spirit and scope of the present disclosure, and thatthey may make various changes, substitutions and alterations hereinwithout departing from the spirit and scope of the present disclosure.

What is claimed is:
 1. A semiconductor device, comprising: a firsttransistor comprising: a first active region, a first gate dielectriclayer over the first active region, the first gate dielectric layercomprising a first concentration of a dipole inducing material, and afirst metal gate stack disposed over and in contact with the first gatedielectric layer; a second transistor comprising: a second activeregion, a second gate dielectric layer over the second active region,the second gate dielectric layer comprising a second concentration ofthe dipole inducing material, and a second metal gate stack disposedover and in contact with the second gate dielectric layer; a thirdtransistor comprising: a third active region, a third gate dielectriclayer over the third active region, the third gate dielectric layercomprising a third concentration of the dipole inducing material, and athird metal gate stack disposed over and in contact with the third gatedielectric layer; and a fourth transistor comprising: a fourth activeregion, a fourth gate dielectric layer over the fourth active region,the fourth gate dielectric layer being free of the dipole inducingmaterial, and a fourth metal gate stack disposed over and in contactwith the fourth gate dielectric layer, wherein the dipole inducingmaterial comprises lanthanum, aluminum, or yttrium oxide, wherein thefirst concentration is greater than the second concentration and thesecond concentration is greater than the third concentration, whereinthe first gate electrode, the second gate electrode, the third gateelectrode and the fourth gate electrode share the same composition. 2.The semiconductor device of claim 1, wherein the first metal gate stack,the second metal gate stack, and the fourth metal gate stack are acontinuous structure.
 3. The semiconductor device of claim 1, whereineach of the first active region, the second active region, the thirdactive region, and the fourth active region comprises a pair of fins. 4.The semiconductor device of claim 1, wherein the first transistor, thesecond transistor, the third transistor, and the fourth transistor aren-type transistors, wherein the first transistor comprises a firstthreshold voltage, the second transistor comprises a second thresholdvoltage, the third transistor comprises a third threshold voltage, andthe fourth transistor comprises a fourth threshold voltage, wherein thefirst threshold voltage is smaller than the second threshold voltage,and the second threshold voltage is smaller than the third thresholdvoltage, and the third threshold voltage is smaller than the fourththreshold voltage.
 5. The semiconductor device of claim 1, wherein thefirst transistor, the second transistor, the third transistor, and thefourth transistor are p-type transistors, wherein the first transistorcomprises a first threshold voltage, the second transistor comprises asecond threshold voltage, the third transistor comprises a thirdthreshold voltage, and the fourth transistor comprises a fourththreshold voltage, wherein the first threshold voltage is greater thanthe second threshold voltage, and the second threshold voltage isgreater than the third threshold voltage, and the third thresholdvoltage is greater than the fourth threshold voltage.
 6. A memorydevice, comprising: a first pull-down transistor, a first pull-uptransistor, a first pass-gate transistor, and a read pass-gatetransistor sharing a first gate electrode; and a second pass-gatetransistor, a second pull-up transistor, a second pull-down transistor,and a read pull-down transistor sharing a second gate electrode, whereinthe first pass-gate transistor and the second pass-gate transistorcomprise a first gate dielectric layer, the first pull-down transistorand the second pull-down transistor comprise a second gate dielectriclayer, the read pass-gate transistor comprises a third gate dielectriclayer, and the read pull-down transistor comprises a fourth gatedielectric layer, wherein the first gate dielectric layer, the secondgate dielectric layer, the third gate dielectric layer comprise hafniumoxide, wherein the first gate dielectric layer, the second gatedielectric layer, the third gate dielectric layer further compriseslanthanum, wherein the fourth gate dielectric layer is substantiallyfree of lanthanum.
 7. The memory device of claim 6, wherein the firstgate dielectric layer comprises a first lanthanum concentration, whereinthe second gate dielectric layer comprises a second lanthanumconcentration, wherein the third gate dielectric layer comprises a thirdlanthanum concentration, wherein the first lanthanum concentration isgreater than the second lanthanum concentration and the second lanthanumconcentration is greater than the third lanthanum concentration.
 8. Thememory device of claim 6, wherein the first gate dielectric layercomprises a first lanthanum to hafnium ratio, wherein the second gatedielectric layer comprises a second lanthanum to hafnium ratio, whereinthe third gate dielectric layer comprises a third lanthanum to hafniumratio, wherein the first lanthanum to hafnium ratio is greater than thesecond lanthanum to hafnium ratio and the second lanthanum to hafniumratio is greater than the lanthanum to hafnium ratio.
 9. The memorydevice of claim 8, wherein the first lanthanum to hafnium ratio is about0.6, the second lanthanum to hafnium ratio is about 0.4, and the thirdlanthanum to hafnium ratio is about 0.2.
 10. The memory device of claim6, wherein a drain of the first pass-gate transistor, a drain of thesecond pull-up transistor, and a drain of the second pull-downtransistor are electrically coupled together.
 11. The memory device ofclaim 6, wherein a drain of the first pull-down transistor, a drain ofthe first pull-up transistor, and a drain of the second pass-gatetransistor are electrically coupled together.
 12. The memory device ofclaim 6, wherein the first pull-down transistor comprises a first pairof fins, the first pull-up transistor comprises a first fin, the firstpass-gate transistor comprises a second pair of fins, the read pass-gatetransistor comprises a third pair of fins, the second pass-gatetransistor comprises a fourth pair of fins, the second pull-uptransistor comprises a second fin, the second pull-down transistorcomprises a fifth pair of fins, and the read pull-down transistorcomprises a sixth pair of fins.
 13. The memory device of claim 12,wherein the first pair of fins and the fourth pair of fins are alignedalong a direction, wherein the second pair of fins and the fifth pair offins are aligned along the direction, wherein the third pair of fins andthe sixth pair of fins are aligned along the direction.
 14. The memorydevice of claim 6, wherein a source of the first pull-down transistor, asource of the second pull-down transistor, a source of the readpull-down transistor are coupled to an electrical ground.
 15. A memorydevice, comprising: a first read pull-down transistor, a first pull-downtransistor, a first pull-up transistor, a first pass-gate transistor,and a first read pass-gate transistor sharing a first metal gateelectrode; and a second read pass-gate transistor, a second pass-gatetransistor, a second pull-up transistor, a second pull-down transistor,and a second read pull-down transistor sharing a second metal gateelectrode, wherein the first pass-gate transistor and the secondpass-gate transistor comprise a first gate dielectric layer, the firstpull-down transistor and the second pull-down transistor comprise asecond gate dielectric layer, the first read pass-gate transistor andthe second read pass-gate transistor comprise a third gate dielectriclayer, and the first read pull-down transistor and the second readpull-down transistor comprise a fourth gate dielectric layer, whereinthe first gate dielectric layer, the second gate dielectric layer, thethird gate dielectric layer comprise hafnium oxide, wherein the firstgate dielectric layer, the second gate dielectric layer, the third gatedielectric layer further comprises lanthanum, wherein the fourth gatedielectric layer is substantially free of lanthanum.
 16. The memorydevice of claim 15, wherein the first gate dielectric layer comprises afirst lanthanum concentration, wherein the second gate dielectric layercomprises a second lanthanum concentration, wherein the third gatedielectric layer comprises a third lanthanum concentration, wherein thefirst lanthanum concentration is greater than the second lanthanumconcentration and the second lanthanum concentration is greater than thethird lanthanum concentration.
 17. The memory device of claim 15,wherein the first gate dielectric layer comprises a first lanthanum tohafnium ratio, wherein the second gate dielectric layer comprises asecond lanthanum to hafnium ratio, wherein the third gate dielectriclayer comprises a third lanthanum to hafnium ratio, wherein the firstlanthanum to hafnium ratio is greater than the second lanthanum tohafnium ratio and the second lanthanum to hafnium ratio is greater thanthe lanthanum to hafnium ratio.
 18. The memory device of claim 17,wherein the first lanthanum to hafnium ratio is about 0.6, the secondlanthanum to hafnium ratio is about 0.4, and the third lanthanum tohafnium ratio is about 0.2.
 19. The memory device of claim 15, wherein adrain of the first pass-gate transistor, a drain of the second pull-uptransistor, and a drain of the second pull-down transistor areelectrically coupled together.
 20. The memory device of claim 15,wherein a drain of the first pull-down transistor, a drain of the firstpull-up transistor, and a drain of the second pass-gate transistor areelectrically coupled together.